Printed circuit boards widely used in electronics and electrical devices are formed with major dimensions of length and width and contain one or more circuits. The thickness of the printed circuit boards varies for many different reasons and directly affects panel flexibility, i.e. there are rigid and flexible circuit boards.
For example, printed circuit boards with multiple conductor planes use a bonded dielectric layer or layers for separation. Holes perforated through the circuit substrate serve a number of purposes including solder terminals for installation into another assembly, plated through hole interconnections between conductor planes and tooling registration holes.
Exposed copper on the finished printed circuit boards must, with few exceptions, be solder coated, a process sometimes termed presoldering or tinning. It is preferable that the solder coating be applied only where needed later and not on all conductor runs. To apply solder selectively, a dielectric, referred to as a cover dielectric or solder mask, is used to cover copper, the normally used conductor, that need not be solder coated. Thus desired copper portions remain exposed and are solder coated, including terminal pads and the like. In other words, the exposed copper on the surface or surfaces of the printed circuit board must be effectively solder coated.
It is also necessary that all holes through the printed circuit board be lined with solder but unobstructed by solder when finished, so as not to obstruct later component lead insertion requirements. The process of removing excess surface solder and clearing the holes of solder, after solder coating the surface conductors, is referred to as solder leveling or leveling.
In other words, printed circuit boards are soldered, inter alia, to maintain solderability for subsequent operations. For economic purposes, such soldering should be accomplished as a mass board coating technique and should provide even coatings on the surfaces and in holes without surface flaws.